On Monday, Yahoo CEO Marissa Mayer announced that her company has agreed to acquire blog hosting website Tumblr for $1.1 billion. The deal has raised copyright concerns, both for Yahoo and for Tumblr’s 100 million users

According to Matt Mullenweg, who founded WordPress, in just one hour 72,000 Tumblr users imported their Tumblr posts to Wordpress after news about the deal was leaked over the weekend. Some Tumblr fans have suggested that Yahoo might change the terms of service so that it owns the rights to all the content in the blogs.

The speculation may be based on fears that history is about to repeat itself. As the New York Times points out, Yahoo’s decision to buy Tumblr is eerily reminiscent of its 1999 acquisition of GeoCities, a popular site which also allowed members of the public to create their own web pages. That deal resulted in a mass exodus of users after Yahoo tried to enforce new copyright rules on the site. Over the next 10 years, GeoCities gradually declined in popularity and Yahoo eventually closed it down in 2009.

The Tumblr acquisition has also created copyright issues for Yahoo. Tumblr’s platform makes it easy for users to violate copyright. Its format focuses on images rather than text and allows users to repost content from other sources at the click of a button.

Earlier this month, adult entertainment company Perfect 10 filed a copyright infringement claim against Tumblr in the Southern District of New York. Perfect 10 claims Tumblr is "willfully ignoring the widespread and uncontrolled copyright infringement pervading its website".

Tumblr is well-known for hosting pornography; the Financial Times reports that web measurement firm SimilarGroup has found that 11% of Tumblr's top 200,000 blogs are adult content.

Websites hosting user-generated content are generally protected from liability for what users upload under laws such as the Digital Millennium Copyright Act (DMCA) and the Communications Decency Act, said Janet Cullum of Cooley.

“These are the laws that allow the web to flourish and grow and develop,” said Cullum, whose practice focuses on trade mark, copyright, advertising and online marketing and privacy issues. “If companies were going to be liable for every infringing photo that gets posted, many of these websites would not exist.”

However, the safe harbour offered by these laws is subject to terms and conditions. For example, under the DMCA, if a website receives notice that it is hosting infringing content, it must remove it in a “timely” fashion. While there is some debate over what constitutes “timely”, Cullum said that companies typically try to respond “in days rather than in weeks”.

While the DMCA specifically addresses liability for copyright infringement, the Communications Decency Act offers more of a blanket protection and is often cited in response to claims for defamation and obscenity.

As social media sites such as Tumblr, Twitter and Facebook continue to increase in popularity, copyright owners are becoming more proactive about enforcing their rights. “We are seeing more and more of that, as the owners of content – particularly in photographs – are becoming more savvy about their rights,” said Cullum.

Copyright owners often turn a blind eye to non-commercial uses, or send a polite letter requesting that the content is removed. “But if you are using it in a commercial context, you risk getting hit with a claim that you have infringed the copyright, or being presented with a demand for licensing fees.”